Electric motor reversing control



Nov. 16, 1948. J. E. REILLY 2,454,207

ELECTRIC MOTOR REVERSING CONTROL Filed June 16, 1945 WITNESSES: 2329 Z84 INVENTOR Patented Nov. 16, 1948 UNITED STATES PATENT OFFICE ELECTRIC MOTOR REVERSING CONTROL Application June 16, 1945, Serial No. 599,839

7 Claims.

This invention relates to electrical systems and, in particular, to control systems for reversible motors of the type utilized in arc furnace regulating systems.

In electronic arc furnace regulating systems, a motor is employed for positioning an electrode, the speed and direction of operation of which is controlled by two sets of electric valves, the biases of which are controlled by a pair of control valves operative in response to the are potential and the current flowing through the electrode. Thus, the operation of the system is primarily controlled by the operation of .the control valves, and if one of the control valves should fail, the system fails to operate in the manner for which it is designed. In particular, if the control valve which controls the operation of the motor to lower the electrode should fail, considerable damage to the furnace and to the system may occur.

An object of this invention is to provide in an electronic arc furnace regulating system for automatically providing protection for the system where failure of the control'valve controlling the lowering of the furnace electrode is encountered.

Another object of this invention is to provide an electronic control system for a reversible motor in which operation of the motor in one direction is prevented upon failure of the control valve which normally controls the operation in the one direction.

A more specific object of this invention is to provide an electronic arc furnace regulating system for automatically preventing the lowering of an electrode when the control valve controlling such operation fails.

Other objects of this invention will become apparent from the following description when taken in conjunction with the accompanying drawing, the single figure of which is a diagrammatic view of the apparatus and circuits of an arc furnace regulating system embodying the teachings of this invention.

Referring to the drawing, this invention is illustrated by reference to an arc furnace I having electrodes I2, I4 and I6 disposed therein to be positioned with respect to a metal charge I8. The electrodes I2, I4 and I6 are connected by conductors 20, 2-2 and 24, respectively, to a source of supply (not shown). Since the .control system and apparatus for each of the electrodes is the same, only the system and apparatus associated with electrode II is illustrated, it being understood that similar control systems are to be provided for the other electrodes.

As illustrated, a reversible motor 26 is utilized for positioning the electrode I4, the electrode I4 being connected to be raised or lowered by operation of the motor. For this purpose, 'a winding drum 28 is disposed to be driven by the motor 26, a flexible cable 30 being connected to the electrode I4 and disposed to be wound upon the drum 28. The motor comprises an armature winding 32 and a field winding 35, the field winding being disposed to be separately excited from a source of supply (not shown).

In order to control the direction and speed of operation of the motor 26, two sets of gas filled electric valves, such as the thyratron valves 36-38 and 4ll42 are utilized. The valves 36, 38, 40 and 42 are provided with anodes 44, 46, 48 and 50, respectively, connected to the terminals of the secondary windings of transformers 52 and 54 as illustrated, the primary windings of the transformers being connected across supply conductors 56 and 58. The valves 36, 38, 40 and 42 are also provided with cathodes 60, 62, 64 and 66, respectively, the cathodes 60 and 62 being connected through conductors 68'I0 and 12, respectively, conductors I4 and I6, resistor 18, armature windings 32, resistor 80, conductor 82, contact member 84 of a contactor 86, and conductor 88 to the center tap of the secondary winding of transformer 52. In a similar manner, the cathodes 49 and 42 are connected by conductors 9G and 92-94, respectively, through conductors 95 and 98, resistor I 00, armature windings 32, resistor I02, conductor i 04, contact member I I36 of a contactor I08, and conductor III] to the center tap of the secondary winding of transformer 54.

The valves 36, 38, 4D and 42 are also provided with grids H2, H4, H6 and II8, respectively. The grids I I2 and I iii are supplied with an alternating current potential from, the supply conductors 5B and 58 through a transformer I20, 2. phase shifting circuit I22 and a grid transformer I24. The secondary winding of the grid transformer comprises two sections I26 and I28, the

- winding I25 having one end connected through a resistor I39 to the grid H 1 and the other end connected through a conductor E32, a part of resistor I34, and the conductor '12 to the cathode E2 of valve 38 whereas the winding I28 has one end connected through resistor 5 36 to the grid H2 and the other end connected through conductor I38, resistor Hi0, and conductors I I, I0 and 68 to the cathode to or" the valve 36.

Alternating-current potential is also supplied to the grids IIS and H8 of valves All and 42, re-

spectively, from supply conductors 56 and 58 through transformer M2, the phase shifting circuit Md and the grid transformer MG which is provided with two secondary windings M53 and I50. The winding Hi8 has one end connected through resistor l52 to the grid lid of valve l2, the other end of winding it being connected through conductor resistor M5, and conductors 96, 9% and $2 to the cathode 66. The secondary winding the has one end connected through resistor E58 to the grid H6 of valve ll} and the other end is connected through conductor Hill, a part or" resistor H32 and conductor to to the cathode ii i.

Potentiometer resistors Mid and 565 are dis posed in circuit relation with the grids ll2l i l and HB-! i3, respectively, of the valves 36-38 and 46- 52, respectively, for controlling the gridcathode potential of the valves in a predetermined manner. Thus one end of the resistor I54 is connected through conductors 68 and ill- 12 to the cathodes iii) and respectively, and a flexible lead its connected to an intermediate tap or resistor Hi l, is connected through resistor 110, conductor H2, resistor lid, and conductor I38 to the secondary winding N8, the conductor H2 also being connected through a part of resistor HE and conductor G32 to the secondary winding E26 of the grid transformer H26. In the same manner one end of resistor ltd is connected through conductors $2 and ti -9E3 to the cathodes I I8 and H6, respectively, of valves 2 and ll respectively, and a flexible lead We connected to an intermediate tap of the resistor ltii, is connected through resistor Hi5, conductor ltd, resistor 32, and conductor i5 3 to the secondary winding M8, the conductor i8il also being connected through a part of resistor and conductor 180 to the secondary winding 55d of the grid transformer MB.

In order to control the direct-current biasing potential for the electric valves and it- 42, vacuum type discharge valves Hi l and 586, respectively, are provided being disposed to normally control the current fiow through the potentiometer resistors ltd and H53, respectively. In addition to the valves 53% and E35, a master controller 538 and a single phase controller Hill are also provided for manual control of the system to effect a raising or lowering of the electrode I4 and a switching contactor 5&2 is provided for automatically controlling the bias of the valves 36 and 38 in case the control valve i8 1 fails.

Only one part of the master controller 388 is illustrated as having segments led, I196, 598, Zllil and 282, it being understood that in practice the master controller is provided with three sets of such segments, one set for each phase, whereby simultaneous adjustment may be obtained in the control system of each phase. The single phase controller ISEl shown in the oil position is of the well known type having lower, off and raise positions and having segments 26 i, Ziiii, 208, 218, 2l2 and 2M positioned to engage cooperating pairs of contact fingers in certain of the positions to establish different circuits.

The control valves lill and Hill are provided with anodes 2H5 and 2E8, respectively, grids 22c and 222, respectively, and cathodes 22d and 226,

Oil

the primary windings of the transformers being connected across supply conductors 5G and 5B.

The cathode 22 5 of control valve E84 is normally connected through a self-biasing resistor 240, a flexible lead 2 32 connected to an intermediate tap of a resistor 2M, conductor W4, resistors M2 and it, conductors l6, l4 and lil, potentiometer resistor led, conductor 2%, segment tilt of controller H33, segment are of c0ntroller Hill, conductor 2%, and smoothing reactor 259 to the other output terminal of the rectifier 232. In a similar manner, the cathode 226 of control valve ltd is connected through a selfbiasing resistor 252, fiexible lead 254 connected to an intermediate tapof a resistor Z55, conductor 82, resistors Bil and ltii, conductors 95 96 and 94, potentiometer resistor E66, conductor 258, segment 208 of controller l'Jll, segment I98 of controller 88, conductors 26D and 262 and smoothing reactor it i to the other output terminal of the rectifier In order to control the biasing potential of the grids 22B and 222 of the control valves I84 and E36, respectively, the grids are so connected that their potentials are controlled in response to the current fiowing through the electrode ill and the arc potential across the electrode arc. Thus, the grid biasing circuit for the valve its extends from the grid 22%? through a grid resistor 2&6, conductor 2538, series connected resistors Eli! and 2H, conductor 2M, segments E96 and 2% of controllers i833 and Hill, respectively, conductors 21B and i6, resistors l8 and H12, conductor lit-l, part of resistor 2M, flexible conductor 2 52 and the selfbiasing resistor 2 5-53 to the cathode 22 The resistors Zlll and 2l'2 are control resistors and are disposed to have a direct-current voltage thereacross proportional to the arc potential and to the flow of current through electrode I4, respectively. For this reason, the resistor 21!] is connected across a rectifier bridge 2'18 which is connected through a transformer 28%, the terminals of the primary winding of which are connected by conductors 282 and 23 to the supply conductor 22 and to the receptacle of the grounded furnace lEl, respectively. The control resistor N2 is also connected across a rectifier 286 which is supplied by a transformer 1288 connected to be energized. by the current transformer ZEN in accordance with the current flowing in conductor 22. The transformers i280 and 238 are preferably of a one to one ratio and are employed primarily to prevent sneak circuits or eiiects on the grids of control valves i8 5 and E86.

As illustrated, the rectifier bridges 21% and 286 are connected in opposition whereby the directcurrent voltages across resistors 210 and 272 are of opposite polarity. Thus, for any variation in the flow of current through electrode i4 and in the are potential, the differential of the directcurrent voltages across the series connected resistors Elli and 2'172 normally controls the directcurrent biasing potential on the grid 220 of control valve I84.

The grid 222 of control valve 986 is likewise connected through a grid resistor 29E, conductor 252, series connected resistors 29% and 96, conductor 298, segments 1194 and 204 of controllers Hi8 and E90, respectively, conductors 308 and 98, resistors Hill and till, conductor 32, a part of resistor 256, flexible lead 254, and the self-biasing resistor 252 to the cathode 225. The control resistor 294 is connected across rectifier bridge 302 which is connected to be supplied through conductors 282 and 284 in accordance with the arc potential. The control resistor 296 is also connected across a rectifier bridge 304, the input terminals of which are connected across the current transformer 230 whereby the direct-current voltage impressed across resistor 296 is proportional to the current flowing through the electrode I4. The rectifier bridges 302 and 304 are so connected across the resistors 294 and 296, respectively, that the direct-current voltages impressed thereacross are of opposite polarity for normally controlling the direct-current biasing potential of the grid 222 of control valve I86.

The switching contactor I92 comprises the contact members 306, 388, 3E0, 3I2, and 3I4 which are disposed to be actuated when the windings 3H3 and 3I8 of the contactor are energized in a predetermined manner. The winding 3I8 is connected across a rectifier bridge 320 which is supplied by transformer 288 in accordance with the current flow through the electrode I4. The contact members 306 and (H2 are break contacts, whereas the contact members 308, 3I0 and 3I4 are make contacts, the break contact members being retained in circuit establishing position until after the make contact members are actuated to circuit closing positions to maintain the winding 3I6 always energized during such a switching operation.

It is to be noted that resistor 256 is connected across the terminals of the armature winding 32 of motor 26 and that a voltage drop appears across resistor 255 which is proportional to the voltage being impressed on the armature windings and that as the polarity of the terminal voltage changes, a corresponding change in polarity of the voltage across resistor 256 is obtained. Thus, the winding 3I6 of contactor I92 is normally disposed to be energized in accordance with the voltage drop across a section of resistor 256 through the circuit extending from an end of resistor 256 through conductors I6 and 216, contact member 306, winding 3 I 6, conductor 322, contact member 3 I 2, and flexible lead 254 to an intermediate tap of the resistor 256.

When the motor is operating in a direction to lower the electrode I4, the potential across resistor 256 is of such a polarity as to energize the winding 3I6 to aid Winding 3I8 to tend to actuate the contact members of contactor I92 in an upward direction. is not obtained until the combined forces of both windings 3I6 and 3| 8 reach a predetermined value, as will be explained more fully hereinafter. On the other hand, if the motor is operating to raise the electrode I4, the potential across resistor 256 is of opposite polarity and windings 3I6 and 318 are energized in opposition to prevent the actuation of the contactor.

When the windings 3I6 and 3I8 cooperate to efiect the actuation of the contactor, contact member 3I4 establishes a circuit extending from conductor 228 through conductor 324, contact member 3I4, and resistors 326 and 240 to the cathodes 224 of control valve I84 to shunt the control valve from the bias control circuit for valves 36 and 38. At the same time, contact members 3I0 and 308 are actuated to circuit closing positions and immediately thereafter contact members 3I2 and 306 are actuated to circuit opening positions, the energizing circuit for the winding then. extending from conductor 228 through conductor 324, resistor 32B, contact member 308, Winding 3I6, conductor 322, contact member 3I0, conductors 330 and 246, segments 200 and 2| 0 of controllers I88 and I90, respec- However, such operation 6 tively, conductor 248 and the smoothing reactor 250 to the other side of the rectifier bridge 232.

The movement ofcontact member 3I4 to shunt the control valve I 84 also establishes a circuit extending from one side of the rectifier bridge 232 through conductors 228 and 32 4, contact member 3I4, resistor 326, flexible lead or conductor 242, part of resistor 244, conductor I04, resistors 102 and I8, conductors 16, I4 and 10, potentiometer resistor I64, conductor 246, segments 200 and 2I0 of controllers I88 and I90, respectively, conductor 248, and the reactor 250 to the other side of the rectifier bridge 232 whereby the current flow through potentiometer resistor IE4 is such as to insure a large negative bias on the grids of valves 36 and 38 to prevent them from conducting.

The resistors 240 and 252 are selected so as to assure a negative bias on the grids of control valves I84 and I86, respectively, when the electrode I4 is in an ideal balanced operating position and the direct-current voltages across control resistors 2'I02'I2 and 294-296, respectively, are so balanced that the difierential or the directcurrent voltages are substantially zero. The potentiometer resistors I64 and I66 in the circuits with the sets of discharge valves 36-38 and 40-42, respectively, are so adjusted by adjusting the flexible leads I58 and I16, respectively, that the drop across the section of the resistors I64 and I66 in circuit with the grids of the valves 30--38 and 40-42, respectively, gives a suflicient dead zone to prevent simultaneous firing of both sets of the discharge valves.

The resistors I8 and I00 in the motor armature circuits controlled by the firing of valves 3638 and 40-42, respectively, are also connected in the grid circuits of the control valves I84 and I86, respectively, and function to automatically limit the starting current of the motor to a safe value for the valves 3638 and 40- 42, respectively. In addition to limiting the starting current of the motor 26, the resistors 78 and 100 also function to protect the valves 36--38 and 4042, respectively, from any overload on the system by so controlling the grid bias of the valves that the current fiow to the motor 26 is within the safe current limits of the valves. The description of the functioning of the resistors 18 and I 00 in the control system may be had by reference to my copending application Serial No. 599,837, filed June 16, 1945, now Patent No. 2,428,583, issued October 7, 1947, and entitled Control systems, in which the valve protecting system is disclosed and claimed.

The resistors 80 and 256 cooperate to permit normal operation of the motor 26 when the valves 36 and 38 are conducting and to provide automatic braking for the motor when the valves 36 and 38 cease to pass current to the motor. Likewise the resistors I02 and 244 cooperate to permit normal operation of the motor 26 when the valves 40 and 42 are conducting and to provide automatic braking of the motor when the current ceases to flow through the valves 40 and 42 to the motor. The system utilizing the resistors referred to for providing automatic braking of the motor is described and claimed in my copending application Serial No. 599,838, filed June it. 18-45. and entitled Control systems.

In operation, assuming that the system is encrgized, the electrodes I 2, I4 and I6 are positioned with respect to the metal charge I8 in the furnace by a manual or automatic operation, such as by operating the master controller I88 or the sin- 7 gle phase controller lilo to the lower position, in which position the contactor it is energized and the contactor its is deenergized. With the contactor 86 energized, the motor 25 is connected in circuit with the electric valves 36 and 38 to effect an operation of the motor in a direction to lower the electrode i l. If the master controller I88 is utilized to effect the lowering of the electrodes, the electrodes l2, it and it are lowered simultaneously, whereas if the individual single phase controller lilii is utilized to effect the lowering of the electrode, the electrodes l2, ill and I6 are independently lowered to contact the metal charge 68, being immediately thereafter withdrawn by operating the master controller I88 or the single phase controller i933 to the off position or to the raised position to establish an are between the electrodes and the metal charge.

On the other hand, if it is desired to automaticall lower the electrodes i2, I l and It as soon as the system is energized and conductors 20, 22 and 2A are connected to a source of power supply and controllers i88 and Hill are in the off position as shown, a potential exists from the electrode it to the metal l8 in the furnace. This potential is at a maximum, and since the electrodes l2, ifi'l and it are not as yet adjusted, there is no current flow. The high value of the are potential impresses a direct-current voltage across each of the control resistors Eli and 29A and since current is not flowing through the electrode i l, the direct-current voltages across resistors 272 and 2% are of zero value. The di rect-current voltage across control resistor 210 places a large negative bias on the grid 220 of control valve i8 1, whereas the direct-current voltage across resistor 2% places a less negative or more positive bias on the grid 222 of control valve The bias thus impressed on the grid 222 of control valve 166 is such that the control valve is rendered more conductive with the result that current flows from the negative terminal of rectifier bridge 2% through reactor 26%, conductors 262 and 26d, segments Hi8 and 288 of controllers I88 and 298, respectively, conductor 258, potentiometer resistor 55%, conductors $4, 96 and 98, resistors I99 and 8%, conductor 32, a part of resistor 256, the flexible lead or conductor 254, resistor 252, cathode 22% and anode 2I8 of the control valve I36, and the conductor 236 to the positive terminal of the rectifier bridge 23 2-. Since the direct-current voltage proportional to the are potential is at a maximum, the conductivity of the control valve 583 is also at a maximum, and maximum current flows through the potentiometer resistor it'd. The flow of current through resistor I65 renders the direct-current biasing potential on the grids H6 and N8 of electric valves to and M, respectively, more negative, whereby the grid-cathode potential of the valves is lowered and maintained at a potential below the critical potential of the valves, and the valves 40 and 42 are blocked with the result that no current flows therethrough to the motor 26.

At the same time the large direct-current voltage proportional to the arc potential impressed across control resistor Elli cooperates with the self-biasing resistor E l-ll to impress a more negative biasing potential on the grid 220 of control valve 584 to decrease the conductivity of the valve I84. Thus, the current flow through potentiometer resistor lo l in the anode-cathode circuit of valve I84 is so decreased that the directcurrent biasing potential for the valves 36 and 38 is so reduced that the resultant grid-cathode potential of valves 36 and 38 rises above the critical potential of the valves, and they become highly conductive to pass current to the motor 26. This conducting circuit extends from the center tap of the secondary winding of transformer 52 through conductor 88, contact member 84 of contactor 86, conductor 32, resistor 80, armature windings 32 of the motor 26, resistor I8, and conductors I6 and M, the parallel conductors 'I068 and I2 to cathodes 6B and 62, respectively, of Valves 36 and 38, respectively, anodes 44 and 46, respectively, of the valves 36 and 38, and from thence to the terminals of the secondary winding of the transformer 52 to so energize the motor 26 as to effect the operation thereof in a direction to lower the electrode I4. Since electrodes I2 and I6 are as yet not in contact with the metal I8, the electrode I4 is lowered to a position where it engages the metal charge I 8.

As the electrode M is lowered, the potential between the electrode i i and the metal charge I 8 decreases with the result that the direct-current voltage across resistors Zlil and 29 i is so decreased that the bias on the grids of valves I84 and I863 is rendered less negative and more negative, respectively. By rendering the biasing potential of the grid of valve I86 more negative, the current flowing through the potentiometer resistor 986 for controlling the biasing potential of the valves to and d2 more nearly approaches the value where the discharge devices to and 42 can be rendered conductive.

As soon as the electrodes I2, I l and iii are actuated to a position Where a circuit is completed through the electrodes in conjunction with the metal charge l8, maximum current flows through the electrode I I and a direct-current voltage proportional to the current is imposed across each of the control resistors 212 and 296, with the result that the differential of the direct-current voltages across resistors 216-272 and 29 1-296 is such as to place a less negative bias on the grid of valve I84 and a more negative bias on the grid of valve I86.

The less negative or more positive bias on the grid 22s of valve 535 renders the control valve I84 more conducting so that more current flows through the circuit extending from the negative terminal of the supply rectifier bridge 232 through the reactor 258, conductor 248, segments 2Ill and 20E] of controllers Hill and 88, respectively, conductor 2%, potentiometer resistor I64, conductors Iii, "i l and I5, resistors 78 and I02, conductor IN, a part of resistor 2, the flexible lead or conductor 2 52, self-biasing resistor 24D, cathode 224, and anode 2M 01" the control valve I84, and conductor 228 to the positive terminal of the supply rectifier bridge 232. The current flowing through that part of potentiometer resistor I 64 in the grid-cathode circuit of valves 36 and 38 renders the direct-current biasing potential on the grids II2 and H4, respectively, more negative to block the firing of the valves 36 and 38 and prevent the flow of current therethrough to the motor 26.

At the same time that the valve I84 becomes conducting, the change in the biasing potential on the grid 222 of control valve I85 is such as to decrease the current passed by the valve I86, with the result that the current flow through the potentiometer resistor I66 is so decreased or becomes zero that the direct-current biasin potential on the grids IIG and H8 of valves 40 and 42, respectively, becomes less negative or more positive, and the valves to and 42 become conducting to pass current to the motor 26. The motor circuit thus established extends from the center tap of the transformer 54 through conductor HG, contact member N of contactor I88, conductor H34, resistor 32, armature windings 32 or" the motor 26, resistor H39, conductors $8 and Elli, the parallel conductors 9t and 9492, cathodes 6i and 3%, respectively, and anodes as and 5! respectively, of the valves. to and re, respectively, to the terminals of the secondary winding of the transformer 54 to so energize the motor as to effect an operation thereof in a direction to raise the electrode 14.

The electrode i4 is thus raised to a position for ideal operation of the arc furnace in which position the directcurrent voltage across each of control resistors 2'33 and 294, which is proportional to the arc potential, and the direct-current voltage across each of control resistors 2'l2 and 2953 and which is proportional to the current flowing through the electrode M, are balanced, and a zero difierential appears across the series connected resistors 215-412 and the series connected resistors 284-286. Under such conditions, the self-biasing resistors 243 and 252 function to maintain sufficient bias on the grids of control valves 184 and 186, respectively, whereby they are sulhciently conducting to pass current through potentiometer resistors its and H36, respectively, that the direct-current biasing potential on the grids of valves -33 and All-t2, respectively, is sufiicient to block the valves from passing current to the motor 28.

If the ideal operating condition for the arc furnace to is changed in any manner whatsoever, the balance of the direct-current voltages across control resistors 219-212 and title-255 is upset with the result that the bias on the grids of control valves i855 and 586 is changed to effect an operation of the motor 26 in a predetermined manner to again establish the required balance between the current the potential across the arc.

If, for example, the electrode M should come in contact with the metal charge it as by reason of the cave-in of the metal, the are potential is reduced to a minimum, and the current flowing through the electrode M is a maximum with the result that the differential of the direct-current voltages across control resistors 2'15} and 272 is of such a polarity as to render the grid of control valve I more positive and thereby render the valve I84 more conductive. As described hereinbefore, when the valve I84 is thus rendered more conducting, the how of current through the potentiometer resistor 164 so changes the gridcathode potential of valves 36 and to render them non-conducting.

At the same time, the difierential of the control voltages across resistors 29% and 295 is such as to place a more negative bias on the grid 222 of control valve 586 to decrease the current flow through the control valve 185. Under such conditions, the direct-current biasing potential on the valves it and 52 is such that the valves are rendered less negative, and current flows therethrough to the motor 25 to cause its operation to raise the electrode Hi.

During the foregoing described operations of the system, it is to be noted that the winding 3i$ of the switching contactor H92 is energized in accordance with the current flow through the electrode l4, and is thus energized in accordance with a component of the direct-current voltages controlling the bias of the control valve I84. At the same time, the winding 3% of the switching contactor W2 is connected by reason of the break contact members 306 and 3H. across part of the resistor 255, which, in turn, is connected across the terminals of the armature windings of the motor 25 whereby the winding 326 is energized in accordance with the terminal potential of the motor 2%. As connected, the polarity of the voltage impressed across the winding 316 when the motor is operating to effect a lowering of the electrode Hi is the same as that impressed across the winding 3l8. If the motor is operating to eiiect a raising of the electrode [4, then the polarity of the voltage impressed across the winding 35% opposes the polarity of the voltage inpressed across the winding 3E8. The physical characteristics of the switching contactor Hill are such that in order to actuate the make contact members 3M, 3M! and 388 to a circuit closing position, the polarities of the voltage impressed across the windings M5 and M3 must be identical, and the winding 3H3 must he energized a predetermined amount depending upon a predetermined current flow through the motor armature 32.

If during the operation of the system as described hereinbefore, the control valve I84 should fail, the failure of the control valve I84 would normally interrupt the how of current through the potentiometer resistor l64, with the result that the bias on the grids H2 and H4 of the valves 36 and respectively, becomes more positive to pass current to the motor 26. Unless such current is controlled, the motor 26 would be operated to drive the electrode l4 through the furnace or the current flow might cause serious damage to the electric valves connected in the system.

As soon as the valve 584 fails, the flow of current from the valves 36 and 38 through the motor 26 is so increased that the potential drop across the resistor 25% becomes so large that the winding SM is energized to cooperate with the winding M8 to actuate the make contact members 3%, Mil and 3M to circuit closing positions and to immediately thereafter actuate the break contact members 3% and M2 to circuit opening positions.

The contact member 3M shunts the control valve its from the circuit with the potentiometer resistor HM, whereas contact members 3! and 38B establish a holding circuit for the switching mechanism 592. The holding circuit extends from the negative side of the supply rectifier bridge 232 through reactor 259, conductor 248, segments 25! and 2% of the controllers I and H38, respectively, conductors 246 and 339, contact member 3M, conductor 322, energizing winding 3H5, contact member 368, resistor 328, and conductors @2 1 and 228 to the positive side of the rectifier bridge 232. This holding circuit maintains the contact member 3M of the contactor H32 in circuit closing position to establish supply circuit for the potentiometer resistor i 54 which extends from the negative terminal of the supply rectifier bridge 232 through the reactor conductor 243, segments Ziil and 200 of controllers Nil and i813, respectively, conductor 2M3, potentiometer resistor 54, conductors l4 and "ill, resistors 78 and H32, conductor 5534, a of resistor 2M, flexible lead or conductor 242, resistor contact member 3M,

conductors 32 i and 223 to the positive terminal of the supply rectifier bridge 232. The

11 flow of current through the circuit thus established places a large negative bias on the grids of the valves 36 and 38 to insure that the valves 36 and 38 will not pass current to the motor 26. While the valve I34 is thus shunted, a new valve may be replaced for the valve which failed.

After the replacement of the control valve I84, the system may be placed on automatic operation again by operating the single phase controller 890 to the lower position in which position segments 2% and 206 interrupt the cir cuits from the control resistors 294296 and TIL-272, respectively, the segment 2m interrupts the circuit to the energizing winding of contactor N38 to interrupt the supply circuit to the motor 26 from the valves ill and 42, and the segment Elli interrupts the circuit from the supply rectifier bridge 232 through the potentiometer resist-or 564, whereby the grids of the valves 36 and 38 become positively biased, and the I valves 36, 38 conduct current to the motor 2&5 in the manner hereinbefore described to effect a lowering of the electrode it. At the same time the segment 2M] interrupts the energizing circuit for the winding 3H3 to deenergize the winding 2H6 whereby the make contact members 3H, 3H] and 308 are actuated to a circuit opening position, and the break contact members 312, 396 are actuated to their circuit closing positions to again connect winding 3H3 to be energized in accordance with the terminal potential of the motor 2% and to again connect the control valve EM in circuit where the valve ltd is disposed to control the bias on the valves 36 and 38.

As soon as the control valve M34 is again connected in circuit relation with the potentiometer resistor I64, the single phase controller I90 is returned to the off position to connect the control resistor 2lli-2l2 and 2941-296 in circuit relation with the control. valves HM and I86, respectively, to control the operation of the control valves, and at the same time the contactor I08 is connected in circuit to actuate the contact member it to the circuit closing position to connect the motor 26 to be supplied from the valves 40 and 42 when the operating conditions of the system so demand.

It will, of course, be appreciated that manual control to raise the electrode Hi can be obtained through operation of the master controller MB to simultaneously effect the raising of the electrodes 52, M and 56 or by a manual operation of the single phase controller lfill to raise the electrode I l independently of the electrodes l2 and N1. For example, the controller 190 may be actuated to the raise position, in which position the segment 2H interrupts the energizing circuit for the contactor 86 to interrupt the supply circuit from the valves 36 and 38 to the motor 26. At the same time, the segments 2M and 20B interrupt the supply circuit from the control resistors 284-296 and 2lt2l2, respectively, and segment 208 interrupts the circuit from the supply rectifier bridge .234 to the potentiometer resistor I66 whereby the valves 4i) and 42 are rendered highly conducting to pass current to the motor 26 to effect an operation thereof to raise the electrode US.

With the system of this invention it is apparent that the control system is protected where the control valve normally controlling the operation to lower an electrode fails making it impossible to damage the furnace and control apparatus as sociated therewith by reason of such failure of the control valve. The protection thus afforded by the system of this invention is instantaneous and automatic and can be obtained in other control circuits by the addition of standard parts without greatly increasing the cost of the equipment.

I claim as my invention:

1. In a control system, in combination, a reversible motor the operation of which is to be controlled, a pair of valve means disposed to selectively connect the motor to a source of alterhating current to control the direction of operation of the motor, each pair of valve means having a grid control circuit including a resistor for controlling the conductivity thereof, a control means disposed to be operated for selectively controlling the flow of current through the resistors for rendering the pair of valve means selectively conductive, each of the resistors being so connected that when current does not flow therethrough the associated valve means is rendered conducting, a source of control voltage responsive to the operation of the motor for effecting an operation of the control means, and switching means disposed for operation in response to a predetermined operation of the motor for rendering the control means inoperative with respect to one of the pair of valve means and to effect a flow of current through the resistor in the grid circuit of said one of the pair of valve means to render said one of the pair of valve means nonconductive.

2. In a control system, in combination, a reversible motor the operation of which is to be controlled, a pair of valve means disposed to selectively connect the motor to a source of alternating current to control the direction of operation of the motor, each pair of valve means having a grid control circuit including a resistor for controlling the conductivity thereof, control means disposed to be operated for selectively controlling the flow of current through the resistors for rendering the pair of valve means selectively conductive, each of the resistors being so connected that when current does not flow therethrough the associated valve means is rendered conducting, a source of control voltage responsive to the operation of the motor for effecting an operation of the control means, and a switching mechanism disposed to be actuated to a switching position in response to a predetermined operation of the motor and to a component of the source of control voltage to render the control means inoperative with respect to one of the pair of valve means and to elfect a flow of current through the resistor in the grid circuit of said one of the pair of valve means to render said one of the pair of valve means non-conductive.

3. In a control system, in combination, a reversible motor the operation of which is to be controlled, a pair of valve means disposed to selectively connect the motor to a source of alternating current to control the direction of operation of the motor, a control means disposed to be operated for rendering the pair of valve means selectively conductive, a source of control voltage responsive to the operation of the motor for eifecting an operation of the control means, and a switching mechanism disposed to be actuated to a switching position to render the control means inoperative with respect to one of the valve means and to render said one of the valve means non-conductive, the switching mechanism having a pair of windings disposed to be energized, one of the windings being disposed to be energized in response to the operation of the motor and the other of the windings being disposed to be energized in response to a component of the source of control voltage whereby the windings cooperate under predetermined operating conditions of the motor to efiect the actuation of the switching mechanism.

4. In a control system, in combination, a reversible motor the operation of which is to be controlled, a pair of valve means disposed to selectively connect the motor to a source of alternating current to control the direction of operation of the motor, a control means disposed to be operated for rendering the pair of valve means selectively conductive, a source of control voltage responsive to the operation of the motor for efiecting an operation of the control means, a switching mechanism disposed to be actuated to a switching position to render the control means inoperative with respect to one of the valve means and to render said one of the valve means nonconductive, the switching mechanism having a pair of windings disposed to be energized, one of the windings being disposed to be energized in response to the operation of the motor and the 1 5. In a control system, in combination, a reversible motor the operation of which is to be controlled, a plurality of pairs of electric valves disposed to selectiveh connect the motor to a source of alternating current to control the direction of operation of the motor, the electric valve-s having grids to be utilized for controlling the conductivity thereof, a grid control circuit including a resistor for each pair of the electric valves, a discharge valve having a control grid therein to be utilized for controlling the bias of the grids of the pair of electric valves associated therewith, a pair of control voltages variable in opposite senses in response to the operation of the motor for impressing predetermined biases on the grids of the discharge valves, and a switching mechanism disposed to be actuated to a switching position in response to a predetermined operation of the motor in one direction and to a component of one of the control voltages to render one of the discharge valves inoperative and simultaneously therewith render the pair of electric valves associated with said one of the discharge valves non-conducting to prevent further operation of the motor in the one direction.

6. In a control system, in combination, a reversible motor the operation of which is to be controlled, a plurality of pairs of electric valves disposed to selectively connect the motor to a source of alternating current to control the direction of operation of the motor, the electric valves havin grids to be utilized for controlling the conductivity thereof, a grid control circuit including a resistor for each pair of the electric valves, a discharge valve having a control grid therein to be utilized for controlling the bias of the grids of the pair of electric valves associated therewith, a pair of control voltages variable in opposite senses in response to the operation of the motor for impressing predetermined biases on the grids of the discharge valves, and a switching mechanism disposed to be actuated to a switching position to render one of the discharge valves inoperative and simultaneously therewith rend-er the pair of electric valves associated with said one of the dis charge valves non-conducting, the switching mechanism having a pair of windings disposed to be energized, one of the windings being disposed to be energized in response to the operation of the motor and the other of the windings being disposed to be energized in response to a component of one of the control voltages whereby the windings cooperate under predetermined operating conditions of the motor in one direction to efiect the actuation of the switching mechanism.

7. In a control system, in combination, a reversible motor the operation of which is to be controlled, a plurality of pairs of electric valves disposed to selectively connect the motor to a source of alternating current to control the direction of operation of the motor, the electric valves having grids to be utilized for controlling the conductivity thereof, a grid control circuit including a resistor for each pair of the electeric valves, a discharge valve having a control grid therein to be utilized for controlling the bias of the grids of the pair of electric valves associated therewith, a pair of control voltages variable in opposite senses in response to the operation of the motor for impressing predetermined biases on the grids of the discharge valves, a resistor connected across the motor to provide a source of potential proportional to the terminal potential of the motor and having a polarity depending upon the direction of operation of the motor, and a switching mechanism disposed to be actuated to a switching position in response to :a predetermined potential across said resistor source as the motor operates in one direction and to a component of one of the control voltages to render one of the discharge valves inoperative and simultaneously therewith render the pair of electric valves associated with said one of the discharge valves non-conducting to prevent further operation of the motor in the one direction.

JACK E. REILLY.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,399,363 Levy Apr. 30, 1946 

